1. Field of the Invention
The present invention relates to a composition comprising a conducting or semi-conducting organic material and a solvent, the composition being suitable for deposition by a printing technique in the manufacture of an organic electronic device such as, for example, an organic light emitting device.
2. Related Technology
A typical organic light-emitting device (OLED) comprises a substrate, on which is supported an anode, a cathode and a light-emissive layer situated in between the anode and cathode and comprising at least one polymeric electroluminescent material. In operation, holes are injected into the device via the anode and electrons are injected into the device via the cathode. The holes and electrons combine in the light-emissive layer to form an exciton, which then undergoes radiative decay to emit light.
Other layers may be present in the device, for example a layer of conductive hole injection material, such as poly(ethylene dioxythiophene)/polystyrene sulphonate (PEDOT/PSS), may be provided between the anode and the light-emissive layer to assist injection of holes from the anode to the light-emissive layer. Further, a hole transport layer made from a hole transport material may be provided between the anode and the light-emissive layer to assist transport of holes to the light-emissive layer.
Luminescent conjugated polymers are an important class of materials that will be used in organic light emitting devices for the next generation of information technology based consumer products. The principle interest in the use of polymers, as opposed to inorganic semiconducting and organic dye materials, lies in the capability for low-cost device manufacturing, using solution-processing of film-forming materials. Since the last decade much effort has been devoted to the improvement of the emission efficiency of organic light emitting diodes (OLEDs) either by developing more efficient materials or device structures.
A further advantage of conjugated polymers is that they may be readily formed by Suzuki or Yamamoto polymerization. This enables a high degree of control over the regioregulatory of the resultant polymer.
Conjugated polymers may be solution processable due to the presence of appropriate solubilising groups. Suitable solvents for polyarylenes, in particular polyfluorenes, include mono- or poly-alkylbenzenes such as toluene and xylene. Particularly preferred solution deposition techniques are spin-coating and inkjet printing.
Spin-coating is particularly suitable for devices where patterning of the electroluminescent material is unnecessary, for example for lighting applications, or simple monochrome segmented displays.
Ink-jet printing is particularly suitable for high information content displays, in particular full color displays.
Other solution deposition techniques for which the present composition is suitable include dip-coating, roll printing and screen printing.
Inkjet printing of luminescent layers of OLEDs is described in, for example, EP 0 880 303.
EP 1 083 775 discloses a composition which can be used in an inkjet printing method. The composition consists of a functional material and a solvent comprising at least one benzene derivative, which has one or more substituents, and three substituents have three or more carbon atoms in total. The following solvents are mentioned: cumene, cymene, cyclohexylbenzene, dodecylbenzene, diethylbenzene, pentylbenzene, dipentylbenzene, butylbenzene, tetralin and tetramethylbenzene. Dodesylbenzene is used as the solvent for red, green and blue luminescent materials in example 2-2 of EP 1083775.
JP 2006-241309 discloses an inkjet composition comprising a diphenylether, for example 3-phenoxytoluene.
WO 01/16251 discloses a formulation for depositing a conjugated polymer layer in a light emitting device. A formulation for depositing a polymer layer in a light emitting device is disclosed where the formulation comprises a conjugated polymer dissolved in a solvent, the solvent comprising at least one substance selected from the groups consisting of terpenes and alkylated aromatic compounds. Preferred polyalkylated aromatic compounds are said to include polyalkyl benzenes such as cymene and isodurenes. According to one embodiment, the solvent comprises at least one aromatic compound substituted with alkyl groups at no less than 3 positions. Dialkyl-benzenes such as diethylbenzene and 1-methyl-4-t-butyl benzene are also disclosed.
WO2006/123167 is concerned with compositions for ink-jet printing conductive or semi-conductive organic material for us in manufacturing opto-electrical devices. It is said in WO2006/123167 that a charge injecting layer may be deposited as a composition comprising a conductive organic material in a high boiling point solvent. PEDOT:PSS is exemplified as a conductive organic material.
WO 2006/123167 also discloses a composition comprising an organic electroluminescent material and a solvent having a boiling point higher than water.
The following high boiling point solvents are disclosed in WO2006/123167: ethylene glycol, glycerol, diethylene glycol, propylene glycol, butane 1,4 diol, propane 1,3 diol, dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone and dimethylsulfoxide.
The key reasons for the interest in ink jet printing are scalability and adaptability. The former allows arbitrarily large sized substrates to be patterned: the latter means that there should be negligible tooling costs associated with changing from one product to another, since the image of dots printed on a substrate is defined by software. At first sight this would be similar to printing a graphic image—commercial print equipment is available that allow printing of arbitrary images on billboard sized substrates [Inca digital website: http://www.incadigital.com/]. However the significant difference between graphics printers and display panels is the former use substrates that are porous or use inks that are UV curable, resulting in very little effect of the drying environment on film formation. In comparison, the inks used in fabricating OLED displays are ink jet printed onto non-porous surfaces, and the process of changing from a wet ink to dry film is dominated by the drying environment of the ink in the pixel. Since the printing process involves printing stripes (or swathes) of ink (corresponding to the ink jet head width) there is an inbuilt asymmetry in the drying environment. In addition OLED devices require the films to be uniform to nanometer tolerance. It follows that to achieve scalability and adaptability requires control of the film forming properties of the ink and a robustness of this process to changes in pixel dimensions and swathe timing.
It is important to use material formulations with which thin polymer films exhibiting excellent uniformity can be produced. In this respect, it is important to use formulations which exhibit the desired properties with respect to surface tension, viscosity, concentration, and contact angle (on the depositing medium and the substrate onto which it is to be deposited), and which preferably also exhibit good solution stability.